Hydraulic systems for directing hydraulic fluid to motors for driving vehicle wheels have already been established in tractor propulsion, for example. Such prior art is commonly dependent upon manual manipulation of hydraulic valves for directing the hydraulic fluid to the particular motor and in particular conditions of timing and fluid pressure. Those systems utilize hydraulic components which permit forward and reverse drive of the vehicle and which also permit selectivity between driving all of the wheels and also driving only some of the vehicle wheels. In those arrangements, the requirement is that a manually operative valve be employed for the various desired settings to enable the wheel drive selectivity mentioned.
The present invention improves upon the prior art by providing a hydraulic system which is arranged to have both manual and automatic control of the fluid flow in the system for the purpose of forward and reverse propulsion of the vehicle and also for driving all of the wheels and thus controlling movement of the vehicle. In these arrangements, consideration is given to the propelling of the tractor on level ground, and up a hill, and also down a hill, and with all of those three conditions also entailing a forward and reverse drive and in two-wheel and four-wheel drive modes. Under some of those conditions, it is desirable to have the vehicle shift from a four-wheel drive mode to a two-wheel drive mode. Also, it is desirable to have some of the vehicle wheels in a non-powered mode, such as with regard to the rear wheels when the vehicle is moving forwardly down a hill. Again, the prior systems commonly necessitate the inclusion of a manually operated hydraulic valve to disengage the four-wheel drive system in order to achieve the desired mode.
Also, the present invention provides the hydraulic system wherein the manually operated selector valve can be only a two-way and two-position valve, rather than say a three-way and two-position valve heretofore utilized.
The hydraulic system of this invention also has the feature of hydraulically engaging and disengaging the drive to the vehicle rear wheels, and to do so both manually and automatically, as desired. In one refinement of this invention, that is achieved by sensing both hydraulic flow and pressure and this invention thereby determines the proper condition to shift from the four-wheel drive to the two-wheel drive. The system arrangements of this invention are arranged to provide for either manual or automatic selectivity within the system to cause the system to alter from a four-wheel drive mode to a two-wheel drive mode. In the automatic mode, there is no requirement for manual, or even electrical, dis-engagement of the four-wheel drive selector valve, such as by means of a reverse switch on the vehicle traction pedal. In the hydraulic flow controlled system, when traveling forward down a slope, the vehicle rear wheels can actually slip and momentarily spin slightly backwards. This system will sense that reverse flow and automatically shift to the two-wheel drive mode. In these arrangements, the system can actually be adjusted to selective hydraulic pressures which can be slightly lower than the pressure required to cause the rear wheels to start to slip and, as such, it is a system which senses the rear motor hydraulic pressure and rear wheel slip.
The hydraulic system of this invention provides for eight different conditions of operation, namely, the four systems of two-wheel drive traveling forward both up and down a grade and traveling in reverse both up and down a grade; and the four systems of four-wheel drive traveling forward in both the up and down grade and traveling in reverse in both the up and down grade.